The chamber for the rifle is in the barrel. So the .300 win heavy bbl is the .300 win heavy bbl no matter which receiver it is screwed to. You can also open the barrel channel to accept the larger diameter barrel but if you switch back it may look awkward on the smaller barrel.

Then there is the business of getting correct head-spacing when switching savage barrels. This must be done correctly or there will be huge safety issues. If you don't know why then you are a long way off from doing it safely.

But we would be glad to help you along that journey. [img]/ubbthreads/images/graemlins/cool.gif[/img]

the switch you ask of would require you to re-chamber the 300 win mag to the 300RUM , unless you have one of the very few Savages that got chambered in the small shank then the two actions have differant thread size.
Now the actions are the same size and one will swap out with the other.
The 300 win barrel could be rechambered for 300RUM but you would have to change the mag well and bolt head around from the 300RUM to the 300Win action because the 300RUM case is fatter and the bolt face larger so it won't feed from the 300Win mag. The next problem with this setup is that you barrel is going to be smaller in diameter around the chamber and that is going to limit you to any hot reloads for safty reasons.
Personaly you best bet would be to just rebarrel the existing 300RUM with a new barrel , this will offer several benifits
1)better quality barrel = better acuracy
2)whatever barrel conture you want = better accuracy
3)longer barrel= better performance in 300RUM
You could order a prefit barrel from Pac-Nor and you could set it up yourself or you could have it rebarreled professionaly and have the barrel setup to do away with the barrel nut (I recomend this one) the end result will be a very accurate LR rig.
It all depends on the money you willing to spend , if you have the funds , get a quality barrel fit to the action properly , and get a good stock the BVSS is ok but very heavy and their are better ones out their , Kevin Rayhill makes one that looks like the McMillian A-5 and is about half the price